Mr Libido Incognito
Actually the oil used with an oilstone is also used as a lubricant, reduces friction making it easier to move the blade and at a more uniform rate.
Actually not, not with a hand held whetstone, the human hand can't move fast enough to generate any significant heat, and oil is actually a very poor heat dispersant. Whetstone oil is strictly for keeping the stone from loading up with metal particles and to wash away loose stone particles, and to lubricate for facilitating ease of motion and to promote uniform motion; helps prevent hand fatigue. Were it a motorized grinding wheel then the high surface speed would certainly cause friction enough to generate exquisite heat, but then the coolant used would be plain water (oils reduce friction so to lessen heat generation but oils do not disperse heat nearly as well as plain old water; to wit engine coolant). In high rate manufacturing facilities most usually water with a small quantity of water soluble oil as an additive to reduce friction and as rust inhibitor (primarily to keep the machine from rusting) and a fungicide (to protect operators), most usually incorporating a particulate filtration device too, to help extend the useful life of the coolant. Also many hand whetstones are of the plain water variety, whereas oil would ruin them. These days manufacturers are more and more moving to using soaps rather than water soluble oils in grinding operations... oil disposal is just too costly.